This invention relates to lasers and in particular to line narrowed lasers.
An important use of electric discharge lasers such as excimer lasers is as a light source for integrated circuit lithography. State of the art KrF and ArF excimer lasers have a natural bandwidth of about 300 pm. Line narrowing systems currently in use reduce this natural bandwidth to about 0.8 pm (FWHM) and about 3 pm (95% integral) for the KrF laser with similar reductions for the ArF laser. Such a prior art system is shown in FIG. 1. In this system, beam 18 is generated in laser chamber 3. The resonance cavity is defined by output coupler 4 (which typically is a partially reflecting mirror) and line narrowing module 6 comprised of a three prism (8, 10 and 12) beam expander, tuning mirror 14 and grating 16 arranged in a Littrow configuration. The wavelength of the beam is monitored by wavemeter 24 which provides a signal for controlling the pivot position of mirror 14 to adjust the wavelength. Wavemeter 24 typically also comprises spectrometer instruments for measuring the bandwidth of the output beam. In some embodiments equipment can be added to the grating to adjust its curvature to improve the bandwidth.
U.S. Pat. No. 5,835,520 assigned to Applicant""s employer teaches a method of bandwidth reduction by reducing the concentration of fluorine in the gas mixture and by corresponding by increasing the reflectivity of the output coupler to compensate for reduced efficiency of the laser.
U.S. Pat. No. 5,856,991 assigned to Applicant""s employer teaches the method of bandwidth reduction by using a line-narrowing etalon output coupler 26 as shown in FIG. 2. This etalon output coupler provides additional spectral selection when its maximum is tuned to the line-narrowing package maximum reflectivity wavelength. This tuning is done by driver 34, controlled by control module 30. This technique is very effective in reducing the bandwidth of the laser, especially 95% integral bandwidth, but it does add complexity and significant cost for the cost of extra parts. Also, tuning of the etalon increases operational complexity.
A known technique for reducing the bandwidth of an output beam of a FIG. 1 laser is to aperture the output beam.
What is needed is the system which can provide additional bandwidth reduction without added complexity of U.S. Pat. No. 5,856,991.
The present invention provides a line narrowed laser system having a spatial filter to filter light at wavelengths greater and/or smaller than a desired range of wavelengths. The laser system is line narrowed with a line narrowing optics including a dispersive element which disperses laser light into wavelength dependent directions. By spatially filtering the resulting beam the bandwidth of the beam can be reduced substantially. There is also a reduction of beam energy as a result of the spatial filtering but this reduction is substantially less than some other known techniques for reducing bandwidths. A preferred spatial filter, for reducing the bandwidth of a line narrowed gas discharge laser, includes two cylindrical mirrors separated by a distance equal to their focal lengths with a slit aperture positioned at the common focal line.
The spatial filtering technique of the present invention is a low cost technique, relatively easy to implement and does not significantly complicate laser operation as compared to some more elaborate systems for reducing laser bandwidth.